1. Field of the Invention
The present invention relates to an optical element using a liquid crystal, and to an optical pickup incorporating such an optical element.
2. Description of Related Art
A liquid crystal that has dielectric constant anisotropy exhibits electric field dependence; that is, it varies its molecular alignment according to the directions of electric force lines. Thus, as the electric field is controlled, the optical characteristics of the liquid crystal can be controlled. This has led to proposals of various optical elements using liquid crystals.
Patent Document 1, listed below, proposes a variable-focus optical system wherein, as shown in FIG. 28, ring-shaped electrodes 101 are arranged concentrically in a pattern of a Fresnel zone plate. This makes it possible to produce, on a liquid crystal panel, a pattern of a Fresnel zone plate of which the spatial frequency can be controlled electrically.
Non-patent Document 1, listed below, discloses an optical element that can vary its focal length by so controlling a voltage as to vary the refractive index distribution of a liquid crystal arranged between electrodes having openings.
Non-patent Document 2, listed below, proposes a liquid crystal lens that is built as a convex lens (having a positive focal length) or concave lens (having a negative focal length) having a liquid crystal sealed between a plano-convex lens or plano-concave lens and a flat-plate glass substrate. In this liquid crystal lens, as the electric field distribution in its liquid crystal portion is controlled, its focal length can be varied in a positive or negative range.
Patent Documents 2 to 4, listed below, disclose optical elements that permit correction of coma. Specifically, Patent Document 2 proposes an optical element wherein, in positions corresponding to openings formed in pattern electrodes (positions deviated from the openings along the optical axis), transparent electrode layers are laid in separate layers from the pattern electrodes. This structure helps eliminate the influence of the potential varying abruptly at the openings in the pattern electrodes, and thus helps enhance the accuracy of aberration correction.
Patent Document 3 proposes an optical element wherein, in openings formed in a transparent electrode, a transparent high-resistance film is laid so that the transparent electrode has apparently no openings at all. This structure helps prevent diffraction of light that occurs between different parts of the transparent electrode, and thus helps obtain a satisfactory optical signal. That is, with no openings in the transparent electrode, there is no place where the potential is locally equal to the reference potential (0 V), nor does diffraction of light occur; thus the loss of light can be minimized.
Patent Document 4 proposes wavefront aberration correcting means and an optical pickup that involve electrodes divided into a shape corresponding to the wavefront aberration distribution attributable to the tilt angle of an optical disc. With this structure, even when the optical disc tilts during reproduction from it, the light spot formed on the recording surface of the optical disc can be so corrected that the magnitude of the wavefront aberration attributable to the tilt is suppressed within a predetermined rage. This ensures satisfactory reproduction.
The patent and non-patent documents mentioned above are as follows:                Patent Document 1: JP-A-S63-249125 (pp. 156 and 157, and FIG. 5)        Non-patent Document 1: Jpn. J. Appl. Phys., Vol. 41, No. 5, p. L571        Non-patent Document 2: Jpn. J. Appl. Phys., Vol. 18, pp. 1679 and 1979        Patent Document 2: JP-A-2001-176108        Patent Document 3: JP-A-2004-334028        Patent Document 4: JP-B-3538520        
The conventional structures described above, however, have the following disadvantages. In the variable-focus optical system according to Patent Document 1, the ring-shaped electrodes 101 have parts of them cut apart (made discontinuous) to leave a space for laying conductors 102 for applying voltages to the electrodes 101. As a result, the electric field distribution in the liquid crystal portion is influenced by the area where the electrodes are cut apart and by the conductors. This causes the pattern of the Fresnel zone plate that is produced on the liquid crystal panel to be deformed from the ideal concentric shape, and may thus lead to degraded optical characteristics.
In the optical element according to Non-patent Document 1, the range in which the focal length can be varied depends largely on the diameter of the openings in the electrodes and on the distance between the electrodes. This results in the focal length being variable only in a range narrower than is expected. With the optical element according to Non-patent Document 2, the focal length can be varied, indeed, but only in a positive range when a plano-convex lens is used and only in a negative range when a plano-concave lens is used.
In any of the optical elements according to Patent Documents 2 to 4, which can correct coma, the electrodes are laid wherever aberration needs to be corrected. Thus, the electrodes are laid in insular segments where coma has peaks. Accordingly, conductors for applying voltages to the electrodes in insular segments need to be laid to cross the optical path. Consequently, the electric field distribution produced by the electrodes is influenced by the conductors.
Thus, in the structure of any of the optical elements according to Patent Documents 1 to 4, the conductors leading to the electrodes are located in the optical path. Disadvantageously, this degrades the characteristics of these optical elements.